I'm picking Hingis here for some of the reasons listed above.
I also question your assessment of Hingis's form at the time of the 1997 French Open vs. at the time of the 1998 French Open.
1997 was when Hingis had her horse riding accident, and she looked really rough in a few of her matches at the French. Gloria Pizzichini was a point away from a 6-3, 5-3 lead against her in the second round, and then there was that bizarre match with Paulus where they traded bagels in sets 2 & 3. That was the match right before Hingis trounced ASV.
OTOH, in 1998, Hingis raced to the titles at Hamburg and the Italian Open, only dropping 1 set across both events (to Venus in the final of the latter event). I'm guessing you're focusing on the surprising loss to Kournikova at the German Open, but Martina followed up on that by easily winning 10 straight sets before Seles found some of her old magic to take her down in the SF. Hingis's play was then pretty spotty for the rest of the season, but I recall her looking pretty good during the clay court stretch.